1. Field of the Invention
The present invention relates to a holding apparatus, optical apparatus, and telescope.
2. Description of the Related Art
A holding apparatus holding an optical element such as a lens generally inserts an optical element having a rotational symmetrical shape with respect to an optical axis into a lens barrel having a cylindrical shape coaxial with the optical axis until one surface of the optical element abuts against the lens barrel, and presses the other surface of the optical element with a fixing member (press ring). As a method of pressing an optical element in the direction of an optical axis, Japanese Patent No. 3345920 has proposed a method of pressing an optical element by screwing male threads (or female threads) on the outer circumferential surface of a press ring into female threads (or male threads) on the inner circumferential surface of the lens barrel, and a method of pressing an optical element using an elastic member such as a spring.
FIGS. 7A and 7B are views each showing the arrangement of a conventional holding apparatus. The holding apparatus shown in FIG. 7A has a structure to press an optical element OE inserted into a lens barrel (main body) LB in the direction of an optical axis with a press ring PR and a shoulder unit BP. More specifically, the optical element OE is clamped and fixed (held) between the press ring PR and the shoulder unit BP by screwing male threads SW formed on the outer circumferential surface of the press ring PR into female threads NT formed on the inner circumferential surface of the lens barrel LB. On the contrary, the holding apparatus shown in FIG. 7B has a structure to press a press ring PR against an optical element OE with a compression spring CS clamped between the press ring PR and a press plate PB.
A conventional holding apparatus determines the position of an optical element with a shoulder unit with respect to an optical axis, and achieves necessary positional accuracy (decentering accuracy) by making the internal diameter of a lens barrel and the external diameter of the optical element equal to each other with respect to a direction (decentering direction) perpendicular to the optical axis. A lateral shift (positional shift in the decentering direction) of the optical element within the lens barrel depends on the processing accuracy of the internal diameter of the lens barrel.
If such a conventional holding apparatus is used to hold a large diameter optical element, however, the processing accuracy is insufficient. It is thus impossible to satisfy specifications required for the holding apparatus to hold the large diameter optical element.
In the astronomical field, for example, an optical element mounted on a reflecting telescope or the like has a larger diameter in recent years, and an optical element having a diameter of about 1 m is used. Such telescope is required to align (in a decentering direction) an optical element having a diameter of 1 m with respect to an optical axis on the order of 1/100 mm (that is, a decentering accuracy of 1/100 mm is required). When applying a conventional holding apparatus to a telescope, therefore, it is necessary to process the internal diameter of a lens barrel with an accuracy of 1/100 mm in order to satisfy the decentering accuracy requirement. Note that it is extremely difficult and impractical to process (finish) the internal diameter of a lens barrel having a diameter of 1 m with an accuracy of 1/100 mm even if a processing apparatus or high technology for enabling high accuracy processing is used. Furthermore, since an optical element with a diameter of 1 m is very heavy (several tens kg), an assembly operation of inserting the optical element into a lens barrel having a clearance of only about 1/100 mm is also extremely difficult, and a special jig, tool, and skill are necessary.